When a biochemical tissue is placed in an even magnetic field space (B0, B0 direction is defined as z) in an MRI apparatus, the magnetic moments of atomic nucleus spins of tissue composing molecules make precession movement around the B0 direction at the natural resonance frequency of each spin. When these spins are exposed to a magnetic field (irradiated radio frequency magnetic field B1) having a frequency near to the resonance frequency from a direction perpendicular to the B0 direction, the net magnetic moment M is rotated (excited) toward the x-y plane, and a net transverse magnetic moment occurs. Thereafter, when the irradiated radio frequency magnetic field B1 is turned off, the magnetic moment being excited is returned (relaxed) to its original state while emitting energy (NMR signal). At this time, the MRI apparatus detects the emitted NMR signal (echo signal) and executes signal processing on the NMR signal to obtain an image of the biochemical tissue.
The MRI apparatus as described above generally images hydrogen protons. Hydrogen is contained in many different molecules in a living body and exists in a living body, so that the resonance frequency of hydrogen protons is slightly different among molecules because the interaction in molecular level is different. For example, in 1.5T MRI apparatus, an echo signal occurring from hydrogen protons in fatty molecules has a frequency which is lower by about 224 Hz than the frequency of an echo signal occurring from hydrogen protons in water molecules. By using this resonance frequency difference, only an image of an echo signal from desired molecules can be obtained.
With respect to clinical imaging, it is required in some cases to image only a signal from water molecules. As a technique for satisfying such a requirement is known a CHESS method of suppressing an echo signal from hydrogen protons of fatty molecules (hereinafter abbreviated as “fatty protons”) by applying a CHESS pulse before actual imaging (non-patent document 1). According to this well-known technique, a radio frequency magnetic field (hereinafter referred to as “RF”) pulse having a fixed magnetic field intensity (in this case, a flip angle is 90° having the resonance frequency of fatty protons, which is a CHESS pulse, is applied to a living body to selectively excite the fatty protons, and then a crusher gradient magnetic field pulse is applied. Accordingly, transverse magnetization of the fatty protons which are selected and excited by the CHESS pulse is subjected to phase dispersion, and the magnetization of the fatty protons is vanished immediately before the actual imaging, whereby the signal from the fatty protons is suppressed.
According to this fatty signal suppressing technique using this CHESS method, if the irradiation intensity of the RF pulse generated by the CHESS pulse is spatially homogenous with respect to fat which is spatially broadly distributed, some fixed suppressing effect can be obtained. However, the chess method has still have an unsolved problem that unevenness of suppression of the fatty signal would occur if the irradiation intensity is spatially uneven. Particularly, it has been reported that the spatial unevenness of the irradiation intensity of the RF pulse is remarkable under high magnetic field (1.5T or more).
Non-patent document 2 discloses a method using an adiabatic type inverting pulse to solve incomplete fat suppression due to unevenness of irradiation intensity of the RF pulse as described above. The method described in the non-patent document 2 does not control a general RE pulse based on only amplitude modulation, but controls an RF pulse which is subjected to frequency modulation (phase modulation) as well as amplitude modulation. A combination of a hyperbolic secant function for amplitude modulation and a hyperbolic tan function for frequency modulation is generally used as modulation functions. Accordingly, a magnetization equilibrium state under the state that desired spins are inverted can be established, and all the desired spins can be kept to be uniformly inverted.    Non-patent document 1: A. Hasse, J. Frahm, et al: H1 NMR chemical shift selective (CHESS) imaging. Phys. Med. Biol. 30, 341 (1985)    Non-patent document 2: “Design of Adiabatic pulses for Fat-Suppression using Analytic Solutions of the Bloch Equation”. MRM 37: 793-801 (1997).